A serial section electron microscope study of an identified Ia afferent collateral in the cat spinal cord

Serial section electron microscopy has been used to examine a horseradish peroxidase (HRP)-labelled group Ia afferent collateral from its entry point in the grey matter to its termination in Clarke's column of the cat spinal cord. A wide range of geometries and myelination patterns were identified along the collateral, including 1) nodes specialized to exhibit a single synaptic bouton, 2) nodes specialized to exhibit two or more synaptic boutons connected by fine, unmyelinated lengths of the collateral, 3) terminal heminodes, along which boutons were separated by unmyelinated branches, and 4) complex arrangements along which myelinated and unmyelinated branches gave rise to one or more boutons. Thirty-six synaptic boutons of varied shape and size were exhibited by this collateral. Previous studies have shown that the geometry, branching, and myelination pattern of an axon play an important role in determining the amplitude and duration of an action potential propagating along that axon. In turn, the amplitude and duration of a presynaptic action potential influence the efficacy of transmitter release. The varied axonal geometries and myelination patterns observed in the present study provide further evidence in support of our previous proposal that there may be considerable nonuniformity in the efficacy of synaptic transmission among release sites arising from the same primary afferent fiber.